Conventionally, such signal processing circuits or systems operate with the following sequence of steps: gating noise impulses out of the amplitude modulated input signal; demodulating the amplitude modulated signals; and then smoothing the demodulated signal after demodulation.
German Patent Publication DE 39 04 505 C2 describes such a system, wherein the smoothing operation takes place downstream of the demodulator stage. The terms “upstream” and “downstream” as used herein have reference to the signal flow or advance through the system from a system input to a system output.
In the German Patent Publication DE 39 04 505 C2 the noise is detected on the wideband high frequency side of the system because on this side the noise impulse has a high amplitude. The known system attempts to gate the noise impulse out of the input signal following the formation of an intermediate frequency upstream of the demodulator stage. The bandwidth of the input signal in the intermediate frequency is still rather large which entails a gating time in the range of a few microseconds. In this context the term “gating a signal” refers to either a short duration interruption of the input signal or a switching off of the input signal. In a physical sense, such signal gating means that under certain circumstances large and in any case undefined noise impulses are replaced by a small defined noise signal. That small defined noise signal is the gating gap. This gated noise passes through a selecting filter normally provided in such circuits. Such a selecting filter in systems for processing amplitude modulated signals has, for example, a bandwidth of about 3.4 kHz.
In such systems the duration of the noise that is the gap width downstream of the demodulator, has increased to several hundred microseconds. Downstream of the demodulator the signal is again gated, for example by a sample-and-hold circuit which produces a gap having normally a duration of about 500 ms. Attempts are then made to again fill up these gaps by more or less complicated interpolation methods. Such gap filling attempts rarely succeed without being heard in the audio signal. As mentioned, in the German Patent Publication DE 39 04 505 C2 a sample-and-hold circuit is arranged in the signal path downstream of the demodulator. The sample-and-hold circuit is supposed to keep the voltage in the signal path constant during the gating in order to achieve a smoothing of the interrupted portion of an audio signal. The just mentioned German Patent Publication describes gating times that are shorter than 250 microseconds.
Generally, signal processing systems that process amplitude modulated signals, encounter the problem that the noise impulses arising at the antenna are superimposed or heterodyned onto the useful signal. As a result, the signal energy at the antenna is increased. A typical example of noise impulses are the pulses generated by the ignition of a combustion engine. These impulses constitute noise that is superimposed on amplitude modulated radio signals. Such noise impulses occur regularly as the fuel in the cylinders of the engine is sequentially ignited, whereby the repetition frequency of these signals is within the range of about 60 Hz to about 600 Hz. The human ear is rather sensitive in this frequency range. As the noise impulse proceeds through the filter stages of the radio receiver, the spectrum and thus also the energy of the noise is reduced. However, the duration of a filter response is coupled to the bandwidth of the filter in accordance with the time bandwidth relationship or law. As a result, an initially large amplitude of the noise impulse can be diminished by filtering only by impairing the impulse width or impulse duration. Thus, the impulse width increases as the amplitude decreases. This phenomenon makes it more difficult to detect the noise impulse and to remove it from the useful signal.